A scanning electron microscope, or SEM, uses electrons to form an image of a specimen. A beam of electrons is produced in the SEM by an electron gun, e.g., by heating a filament. The electron beam follows a path through the column of the microscope and is focused and directed toward the specimen to be imaged using a series of electromagnetic lenses and apertures. When the electron beam strikes the specimen, a number of signals are generated. The different signals can be used to generate different types of images of the specimen.
One signal comes from what are known as backscattered electrons. This signal is obtained by collecting and analyzing those incident electrons that “bounce” off after interacting with the nuclear and electronic potentials of the atoms of the specimen, substantially back toward their source. Thus, the name backscattered electron. Because these electrons are moving so fast, they can penetrate relatively far into a specimen, and after scattering from sites well below the surface they can escape the specimen. In an SEM, a detector is placed in the path of backscattered electrons, and the resultant signal is used to create an image of the specimen. The abundance and energy of backscattered electrons varies with the specimen's local atomic number. As a result, regions of higher average atomic number appear brighter than those of lower average atomic number elements. Thus, backscattered electrons can be used to get an image that shows the different elements present in the specimen.
Another signal comes from what are known as secondary electrons. A small amount of energy from the incident (beam) electrons couples to the electrons already present in the specimen by electromagnetic interaction. This raises the energy of some specimen electrons to a level that enables them to overcome the specimen's work function, and escape the surface of the specimen with a relatively small kinetic energy, on the order of 5 eV. These electrons are called secondary electrons. Because the energy coupled from the primary beam is small, only specimen electrons that are near the surface of the specimen, typically within 10 nm, can become secondary electrons; specimen electrons from deeper below the surface lose the energy absorbed from the beam before reaching the surface. Thus, images formed by secondary electron detection (SED) can achieve high resolution, as they represent the surface only and not the bulk of the specimen. Secondary electrons are collected by a positively charged detector for production of an image. As is well-known by microscopists, the physical processes involved in SED are highly sensitive to the topography of the specimen. Thus, SED images give contrast according to the surface shape of the specimen, analogous to a conventional photograph of an object illuminated from a point source.
There are several well known techniques used to prepare a specimen prior to examining the specimen in a SEM. Among these techniques are etching, coating and plasma cleaning.
Etching involves the selective removal of a portion or portions of the specimen by one of several processes. Etching can be useful when there is a desire to examine a buried feature of a layered structure such as a microchip. In this case, the top layer or layers can be etched away to expose a buried feature for examination. In addition, as described in more detail below, etching can be used to planarize or smooth out a surface of a specimen, such as the surface of a trench cut into a specimen using a focused ion beam generated, for example, from gallium. In this case, a lower energy ion beam generated from argon can be used to etch and thus planarize the trench surface.
Finally, one important use of etching is in the semiconductor industry, where it is often desired to examine a cross-section of a multi-layer structure made of different materials. When created, however, these cross-sections have little or no topography, and thus cannot be effectively imaged using the secondary electron mode of an SEM. Depending on how the etching process is carried out, different materials will etch selectively, meaning at different rates. By using a selective etching process on a specimen consisting of a cross-section of a multi-layer structure made of different materials, the layers will etch at different rates, resulting in the different layers having different heights. Then, the specimen can be imaged using the secondary electron mode of the SEM, with the topography created by the selective etching providing information on, e.g., the boundaries of each layer in the specimen.
Many etching methods are well known in the art. These methods include ion beam etching, abbreviated IBE, reactive ion beam etching, abbreviated RIBE, chemically assisted ion beam etching, abbreviated CAIBE, and plasma etching, otherwise known as reactive ion etching, abbreviated RIE. As the names suggest, IBE, RIBE and CAIBE all utilize an ion beam in the etching process.
In IBE, an ion beam composed of an inert gas such as argon is generated by an ion beam source, otherwise known as ion gun, and is aimed at a target specimen. The ion beam removes material from the specimen by momentum transfer. In particular, the impinging ions of the ion beam knock atoms out of the target specimen. In IBE, there is a small degree of selectivity, meaning different materials are removed at different rates, because the efficiency of momentum transfer from the ion beam depends on the atomic mass of the target. There is also a high degree of directionality, or anisotropy, because the ions impinge on the specimen from a particular direction. This anisotropy can be used to obtain particular desired results for sample preparation. For example, to produce a smooth, level surface, the ions can be made to impinge on the specimen from a direction nearly parallel to the desired surface. This type of etching is known as planarization and is characterized by having the ion beam impinge on the specimen at lower angles of incidence. Features protruding from the surface will be eroded more quickly than areas in the surrounding plane, and so there is a leveling effect due to the anisotropy. Using IBE for planarization to produce a smooth, level surface is also known as “ion milling.” Etching at higher angles of incidence results in topographical enhancement of the specimen because the selectivity of the etching increases as the angle of incidence increases.
In RIBE, a reactive gas, such as CF4, is used by the ion gun to generate the ion beam. As a result, in addition to momentum transfer, a chemical reaction effects the removal of material from the specimen target. The chemical reaction adds a higher degree of selectivity to the process, because different chemical reactions occur with different components of the specimen, and in general these different reactions can have very different rates.
One problem with RIBE is that the reactive ions can also react with the materials of which the ion gun is constructed. This causes corrosion and early wear-out of the gun. CAIBE is somewhat of a hybrid between IBE and RIBE that avoids this problem. In CAIBE, a reactive gas flow, such as iodine, is aimed at the specimen target by a neutral device such as a hose or nozzle. At the same time, an ion beam composed of an inert gas is aimed at the specimen target. The impingement of the ions of ion beam on the surface of the specimen facilitates the chemical reactions caused by the impingement of the reactive gas on the surface of the specimen, thereby providing a more effective selective etch than IBE alone. The problems of RIBE are avoided because the reactive gas is not ionized in the ion gun.
Historically, IBE techniques have been used for Transmission Electron Microscopy (TEM) sample preparation. Due to recent advances in SEM technology, IBE techniques are becoming more applicable for SEM sample preparation. Sample geometry becomes a limitation when trying to adapt IBE technology from TEM to SEM. TEM samples are very consistent in size, whereas SEM samples geometries vary greatly. Many IBE devices are designed to only accommodate the consistent sample size of a TEM sample. When adapting IBE technology to SEM, a system to detect and adjust for variations in sample geometry, or more specifically, the overall height of the sample becomes necessary for practical use of the device.
In plasma etching (RIE), the specimen is exposed to a chemically reactive plasma. Depending on the gaseous species used to generate the plasma, different chemical reactions will be included and selective etching will occur. Many methods are well-known for generating a plasma for plasma etching. One class of plasma etching equipment places the specimen in a gap between two substantially planar, substantially parallel electrodes. Gas is introduced into the gap at a low pressure, for example 1 torr, and the electrodes are connected to the terminals of an alternating voltage source, resulting in an alternating electric field within the gap. The electric field couples energy to electrons of the gas, ionizing some fraction of the gas molecules, thereby forming a plasma within the gap. Because the coupling is primarily electrostatic, this technique is known as “capacitive discharge” plasma. The plasma can contain the original species in the feed-gas, as well as many other combinations of the atoms of the original feed-gas species. The plasma can contain these species as neutral molecules as well as positive- or negative radicals. In the context of etching, any molecule or fragment thereof with a net charge is called an “ion.” Ions are accelerated out of the plasma by the electric field of the plasma sheath toward any material surfaces, including the electrodes and the specimen. Neutrals permeate the enclosure by way of diffusion. The ions and/or neutrals can cause a selective etching result because of their greater or lesser disposition to react chemically with the materials of the specimen. In addition, the ions can cause a directional or anisotropic etch result because they are accelerated toward the specimen in a particular direction. These two characteristics taken together make RIE especially useful for introducing topography to cross-sectional samples composed of layers of different materials. The selectivity tends to etch the different materials at different rates, resulting in topographic relief. The anisotropy helps to preserve sharp edges, by reducing the rate of lateral etching, while enhancing the rate of vertical etching. These effects can be enhanced by many techniques including the varying of the relative size of the parallel plates and adding an independent DC source.
Another class of plasma etching equipment uses an alternating electromagnetic field to couple energy to the electrons of a plasma. This is known as inductively coupled plasma, or ICP. The generation of ions and neutral species is similar to that which takes place in the above example. However, the sheath voltage of an ICP is generally much lower than the sheath voltage of a capacitive discharge, and so the ions generally exit the plasma with less speed, resulting in lower anisotropy. In more advanced ICP etchers, the degree of anisotropy can be controlled by adjusting the electrostatic potential of the specimen relative to that of the plasma interior.
Still more similar methods of plasma etching have been developed, particularly for application in semiconductor processing. These methods and the equipment used therefor are known in the field as “dry etching” systems, in contrast to methods and equipment that use acids, for example, which are known as “wet etch” systems.
Since an SEM uses electrons to produce an image, most conventional SEMs require that the samples be electrically conductive. If the specimen is made of a non-conducting insulating material, the impinging electrons of the SEM are not conducted away from the material and will accumulate on the surface of the specimen and cause charging effects that disturb the trajectories of subsequent beam electrons and reduce the quality of the image. In order to image a non-conductive specimen made of an insulating material such as a ceramic or plastic, the specimen must be coated with a thin layer of a conductive material before being imaged in an SEM. Some commonly used conductive materials are carbon, platinum, palladium, gold, gold-palladium, chromium, aluminum and tungsten. The particular conductive material chosen depends on the application. Typically, specimen coating is performed in one of two ways: by thermal evaporation methods or by sputtering methods.
A number of thermal evaporation methods are well known in the art, including resistive heating and electron beam evaporation. In thermal methods, the specimen is placed in a vacuum chamber and evacuated to e.g., 10−3 to 10−5 torr, and the coating material to be applied to the specimen is heated within the chamber. In resistive heating methods, the coating material is heated by placing it in contact with an electrical conductor, through which an electrical current is passed, causing heat. Typically, the conductor is in the form of a tungsten boat. In electron beam evaporation methods, electrons are emitted from a filament and accelerated toward the source of coating material to be evaporated. The impact of the electrons impinging on the source material causes heat within the material. By either method, heating the material causes thermal evaporation of the material. The sample to be coated is located so as to experience a flux of the evaporated material on its surface. Because the sample is thermally cool, the evaporated material condenses on its surface and forms a coating.
Several sputtering methods and devices for coating samples are well known in the art. Two such well known methods are ion beam sputtering and magnetron sputtering.
In ion beam sputtering, an ion beam, unusually composed of an inert gas, is aimed at a target consisting of the conductive material. The ion beam removes material as in IBE or ion milling. The specimen is arranged so that some of the removed material will impinge upon it and stick, thereby coating the specimen with a conductive material.
In magnetron sputtering, a magnetically confined, DC plasma is generated that results in a high flux of ions, usually inert, that impinge on the conductive target and remove material by momentum transfer. As in the case of ion beam sputtering, the specimen is arranged so that some of the removed material will impinge on the specimen and stick, forming a coating.
As is well known in the art, a number of parameters involved in the sputtering process control the appearance of the final coating. These parameters include specimen temperature, distance of the specimen from the target, manipulation of the specimen as it is being coated, including rotation, rocking and tilting thereof, target orientation, primary ion energy, vacuum level and target material.
Another essential component of any coating process is the ability to monitor the thickness of the coating material being applied to the specimen surface. The most widely used method uses a crystal oscillator that is placed in the coating chamber near the specimen to be coated. The resonant frequency of the crystal is measured and is a function of how much material has been applied to the surface of the crystal; this in turn can be related through geometry to the quantity of material deposited on the surface of the specimen. The coating process may be automatically controlled such that once a desired thickness of coating material is applied, the coating process is automatically terminated.
One problem that adversely affects the quality of SEM analysis is hydrocarbon contamination of the specimen. This contamination can occur as a result of poor operator handling techniques during the preparation process, such as touching the specimen with an ungloved hand. Other contamination may result from subjecting the specimen to a preparation process that utilizes an oil diffusion pump or a turbomolecular pump backed by a vacuum pump that utilizes oil in its pumping path whereby backstreaming of oil will lead to contamination, the use of hydrocarbon based solvents and adhesives in the preparation process, storage or exposure of the specimen to ambient conditions, and repeated exposure to the SEM vacuum system which may contain oil vapor which has migrated up the electron optics column from a vacuum pump or has entered the chamber through its exposure to ambient conditions. Although contamination of specimens may consist mainly of hydrocarbon compounds, other types of contamination, such as oxides or particulates, can be present. Furthermore, contamination of the specimen surface and of the target can have a detrimental effect on the quality of the coating deposited by a sputtering coating method. Such contamination can lead to adhesion difficulties for the coating material, unevenness of the coating, and possibly the formation of unwanted inter-metallic species. Contamination of the sputtering target can lead to the deposition of unwanted compound materials rather than the monatomic metallic species.
Moreover, a specimen may become contaminated when it is transferred in the ambient environment from one processing device to the next while being prepared for microscopy. For example, a specimen may be so contaminated when transferred from a stand alone plasma cleaning device to a separate stand alone coating or ion milling device.
There are two common cleaning methods that are used in sputtering systems as a quick means of cleaning the specimen. In the first of these methods, used with ion-beam sputtering, the sputtering ion source is re-aimed at the specimen, and the sample is then ion milled for a short period of time. This will knock off unwanted contamination from the surface of the specimen. However, the unwanted contamination is then free to redeposit back onto the specimen or perhaps the sputter-coating target. Furthermore, sputtering of the specimen surface can lead to etching effects. The second of these methods is used in magnetron sputtering systems. In this method, the polarity of the target is reversed so that the ions are accelerated toward the specimen. That is, the roles of target and specimen are reversed. This also may result in etching of the specimen surface, particularly for softer materials.
An alternative cleaning method and solution is described in Fischione, U.S. Pat. No. 5,633,502, entitled “Plasma Processing System for Transmission Electron Microscopy Specimens and Specimen Holders”, the disclosure of which is incorporated herein by reference. Fischione discloses a plasma processing method and apparatus in which a low energy RF plasma is preferably used to remove contamination, mainly in the form of hydrocarbons, from specimens. The system comprises a vacuum system, a plasma chamber into which the specimen and the specimen holder are inserted, a housing having an access port with removable inner sleeve components, and a RF power supply which is coupled to the plasma chamber and enables both the generation and maintenance of the plasma. To commence processing, the vacuum system is engaged for the evacuation of the plasma chamber for subsequent formation of the plasma. Plasma formation is preferably initiated through the coupling of an oscillating field to the plasma chamber. Many process gas mixtures can be used, including a mixture of a noble gas and an oxidant. A 25% oxygen and 75% argon mixture is preferred. The oxygen chemically reacts with carbonaceous substances on the specimen and converts them to volatile species such as CO, CO2 and H2O. The argon dilutes the oxygen and thereby simplifies safety issues in gas handling.